What is the best way to ground the frame of a photovoltaic module?
This is an apparently simple question, with a complex answer. When exposed to sunlight, PV arrays can generate dangerous levels of voltage (up to 600 volts) and current. The frames of these modules must be effectively and continually grounded to earth to prevent electrical shocks and to reduce fire hazards from stray ground-fault currents.
When ground faults occur in a PV system, these currents may circulate indefinitely under certain conditions. Unlike a ground fault in an AC power system, which is interrupted immediately, a DC ground fault may exist whenever the module is illuminated. In larger commercial (nonresidential) systems, the ground-fault detection system does not interrupt these currents. The connections that are used for grounding PV modules may have to be as robust as those used for the circuit conductors.
Grounding PV modules is complicated by several factors. A typical aluminum-framed PV module has a clear or colored anodizing on its surface that must be removed or breached for good electrical contact. When these coatings are removed, the bare aluminum will oxidize very quickly (in seconds) and build up an insulating film that also prevents good electrical contacts. Plus, the copper equipment-grounding conductor must not come directly into contact with the aluminum surface, since galvanic corrosion between these two dissimilar metals will occur, eventually resulting in a failed connection.
Unfortunately, although inspectors have been providing examples of failed grounding methods and devices, the grounding hardware and instructions provided by PV module manufacturers have not yet been tested and evaluated by Underwriters Laboratories (UL). Under pressure from the PV industry and the electrical inspection community, UL now has undertaken a major investigation of PV module grounding. However, the results of the UL investigation are not yet known.
Based on discussions with grounding-lug manufacturer FCI-Burndy and using utility company procedures to connect copper wires to aluminum busbars in an outdoor environment, I'm employing the following procedure to make equipment-grounding connections to module frames. These procedures are used only when they do not directly contradict manufacturer's instructions provided with the listed module.
At one of the marked grounding points on the module frame, an abrasive material like emery cloth is used to remove the clear coat, anodizing, and aluminum oxide from the surface where the ground lug will contact the aluminum surface. Immediately, a thick layer of antioxidant compound is applied to the exposed aluminum surface. Any excess compound will be squeezed out when the lug is bolted in place. A tin-plated, solid-copper, direct-burial-rated lay-in lug is used to connect a copper conductor to the exposed aluminum frame.
A bolt, nut, two flat washers, two split-lock washers and a Belleville (cupped spring) washer are used to bolt the lug to the frame. The flat washers are used to prevent the hard
steel split-lock washers and Belleville washers from digging into the relatively soft copper and aluminum. The split-lock washers and the Belleville washer are used to maintain the assembly under the correct tension. Use a calibrated torque screwdriver set to 12 to 15 inch-pounds (depending on the type of bolt) to ensure a reliable connection. A copper conductor (generally from #12 to #4) is attached to this lug. The size of the conductor depends on the electrical grounding requirements, the need for physical protection, and the requirements of the local inspecting agency.
Other Questions or Comments?
If you have questions about the NEC or the implementation of PV systems that follow the requirements of the NEC, feel free to call, fax, e-mail, or write me at the location below. See the SWTDI Web site (below) for more detailed articles on these subjects. The U.S. Department of Energy sponsors my activities in this area as a support function to the PV industry under Contract DE-FC 36-05-G015149.
John Wiles ([email protected]) works at the Southwest Technology Development Institute, which provides engineering support to the PV industry and provides industry, electrical contractors, electricians, and electrical inspectors with information on code issues related to PV systems. An old solar pioneer, he lives in his utility-interactive PV-powered home in the suburbs.
Southwest Technology Development Institute, New Mexico State Univ., Box 30,001/ MSC 3 SOLAR, Las Cruces, NM 88003 • www.nmsu.edu/~tdi
The 2005 National Electrical Code and the NEC Handbook are available from the National Fire Protection Association (NFPA) • 800-344-3555 or 508-895-8300 • www.nfpa.org
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